The present invention relates to a method for anti-slip control, in particular for traction control, in a motor vehicle, in which, in a first operating mode, a braking force is applied individually to each driving wheel with a tendency to spin to reduce its slip; and in which, in a second operating mode, an output quantity of the driving motor is additionally reduced if two driving wheels on the same axle show a tendency to spin. The present invention further relates to a device for carrying out the method.
In an anti-slip control device for a motor vehicle, the rotation of the driven wheels is ordinarily monitored by detecting the velocities of the driven wheels and comparing them with each other and/or with the velocity of a non-driven wheel. The anti-slip control system can be designed exclusively as a traction control system, as described, for example, in SAE paper 87,03,37 entitled xe2x80x9cASRxe2x80x94Traction Controlxe2x80x94A Logical Extension of ABSxe2x80x9d. Alternatively, however, the anti-slip control system can be provided as part of a vehicle stability control system, which is used to control vehicle yaw rate by interventions in the vehicle brakes and motor, and in which an anti-slip controller is implemented as a secondary controller. The basic functions of a vehicle stability control system are described, for example, in the article entitled xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d [FDRxe2x80x94Bosch Vehicle Stability Control System] published in Automobiltechnische Zeitschrift (ATZ) 96, 1994, Volume 11, pages 674-689.
According to a traction control device for a motor vehicle known, for example, from German Published Patent Application No. 29 14 165, differences between the velocities of announced and non-driven wheels are used to determine the tendency of the driving wheels concerned to spin. If a tendency to spin is detected, the driving wheel concerned is braked individually. The buildup of braking force can be dependent on the rotation of the driving wheel with a tendency to spin, in particular, it can be dependent on its slip. This operating mode, in which an individual braking force is applied to each driving wheel with a tendency to spin or already spinning independently of the other driving wheels, is also known as select-high (SH) mode.
In a further operating mode, also known as select-low (SL) mode, which is used primarily when two driving wheels on the same axle show a tendency to spin, the driving wheels concerned are not stabilized by an individual brake intervention at each wheel, but rather by a synchronous brake intervention with additional engine throttling.
During a starting motion, therefore, the known traction control system initially applies a braking action to a driving wheel when the latter shows a tendency to spin, while it switches from select-high to select-low mode if the second driving wheel also shows a tendency to spin. In select-low mode, drive slip at the two driving wheels is additionally controlled at least by reducing the driving torque of the motor vehicle driving motor. The driving torque of the driving motor is reduced until at least one of the driving wheels starts stabilizing.
In a traction control system of this type, it is especially important to switch between operating modes during a starting motion, in particular to switch from select-high to select-low mode. Indeed, if the driving wheels are on a roadway with varying coefficients of friction (known as a xcexc-split roadway), the switch from select-high mode to select-low mode takes place even if the instability of the driving wheel running at a high coefficient of friction lasts only a short time.
Especially when starting on xcexc-split roadways, braking force builds up at the first spinning driving wheel according to the slip detected there. A high driving torque that causes the wheel to spin generates a large level of drive slip at this wheel, resulting in high braking force being applied to this wheel in connection with the traction control system. This can cause instability in the wheel with a high coefficient of friction if the latter can no longer remain in contact with the roadway due to the torque now being transmitted by the breaking force buildup at the spinning wheel. However, this instability is normally of only short duration. Yet switching to select-low mode would reduce driving torque, so that a significant, unwanted drop in traction can be felt when starting.
An object of the present invention is therefore to provide a method for anti-slip control, in particular for traction control which can be used to avoid, in a particularly reliable manner, unwanted switching from the first to the second operating mode. Another object is to provide a device for anti-slip control in a motor vehicle that is especially suitable for carrying out the method.
With regard to the method, this object is achieved according to the present invention with a switch from the first to the second operating mode taking place no earlier than the end of a first waiting time that is selected in proportion to the difference between the braking forces applied to the two driving wheels with a tendency to spin and occurs after the tendency to spin of the second driving wheel is detected.
The present invention is based on the idea that unnecessary, and thus unwanted, switching from the first to the second operating mode can be avoided with particular reliability by suitably filtering the processing of the events triggering a switch. In this regard, only those events in which a change in roadway properties is detected with adequate reliability should be taken into account. An especially easy way to achieve such filtering is to introduce a waiting time during which a brake intervention is applied as early as the first operating mode (select-high mode) to both driving wheels with a tendency to spin before finally switching to the second operating mode (select-low mode). The waiting time should be specifically coordinated to detect typical roadway properties. This takes into account the fact that extremely variable roadway coefficients of friction at the two driving wheels is a highly probable indication of a xcexc-split roadway, which means that both driving wheels can remain unstable for only a short period of time without requiring switching to select-low mode. Only minor differences in the coefficients of friction at the two driving wheels, on the other hand, are more likely to mean a roadway with a generally low coefficient of friction, which can make it necessary to switch to select-low mode. In selecting the waiting time length, therefore, the difference between the braking forces that act upon the two driving wheels and make it possible to determine the differences in the coefficients of friction, is suitably taken into account.
In the second operating mode, or select-low mode, the driving motor""s output, rotational speed or another suitable quantity can be reduced as the output quantity of the driving motor. In this operating mode, however, the driving torque of the driving motor is suitably reduced as the output quantity of the driving motor.
In selecting the first waiting time, the amount of the difference between the brake pressures acting on the two driving wheels with a tendency to spin is advantageously taken into account as the quantity that is characteristic for the active braking forces. To determine the first waiting time, this difference, measured in bar, is suitably multiplied by a conversion factor of approximately 40 ms/bar, yielding, for example, a first waiting time of approximately 280 ms with a pressure difference of 70 bar.
An especially favorable performance of the traction control system, with only minor traction losses, can also be achieved by first switching from the first to the second operating mode and then switching back to the first operating mode, or select-high mode, as early as possible, taking safety considerations into account. In an especially advantageous embodiment, this is accomplished by a switch from the second to the first operating mode being carried out no earlier than the end of a second waiting time that is selected as a function of the braking force applied and occurs after the tendency to spin of a driving wheel is last detected.
An initial prerequisite for switching back is the passage of a certain minimum time in select-low mode without another deviation being detected. Switching back is possible at the end of this safety period. In determining the safety period, or second waiting time, the braking force applied in select-low mode is taken into account, and thus the information available about the road conditions. A pre-selectable maximum time of, for example, approximately 1,300 ms, which is reduced in proportion to the braking force applied, is suitably selected as the second waiting time. The maximum time is suitably reduced by the brake pressure applied, measured in bar, and multiplied by a conversion factor of approximately 20 ms/bar, thus yielding, for example, a second waiting time of approximately 300 ms with a brake pressure of 50 bar.
With regard to the device for anti-slip control in a motor vehicle, the object mentioned above is achieved with a control unit which, in a first operating mode, outputs actuating signals for setting an individual braking force at each driving wheel with a tendency to spin to reduce its slip, and which, in a second operating mode, outputs additional actuating signals to reduce an output quantity of the driving motor if two driving wheels on the same axle show a tendency to spin; and the object mentioned above is also achieved with a switching module that switches the control unit from the first to the second operating mode no earlier than the end of a first waiting time that is selected in proportion to the difference between the braking forces applied to the two driving wheels with a tendency to spin and occurs after the tendency to spin of the second driving wheel is detected.
The switching module suitably sets, as the first waiting time, the amount of the difference between the brake pressures, measured in bar, acting on the two driving wheels with a tendency to spin, multiplied by a conversion factor of approximately 40 ms/bar.
According to a particularly advantageous embodiment, the switching module switches the control unit from the second to the first operating mode no earlier than the end of a second waiting time that is selected as a function of the braking force applied and occurs after the tendency to spin of a driving wheel was last detected, with the switching module according to a further suitable embodiment selecting, as the second waiting time, a pre-selectable maximum time, minus the brake pressure (bar) applied that was multiplied by a conversion factor of approximately 20 ms/bar.
The particular advantages achieved by the present invention lie in the fact that, with especially simple means, it reliably takes into account the roadway properties when switching between the operating modes, due to the waiting time selected as a function of the braking forces applied. By additionally taking into account the brake pressure level of the driving wheel already being controlled, it is also possible to determine the current differences in the roadway coefficients of friction.
The short-term instabilities of both driving wheels, which occur, in particular, with xcexc-split roadways, do not immediately trigger a switch from the first (SH) mode to the second (SL) mode. Instead, events of this type, which do not require switching to select-low mode due to the short-term nature of the instability, are filtered out of the possible switch triggers by suitably selecting the first waiting time. Furthermore, switching to select-low mode is not unnecessarily delayed, since only a first waiting time that is in proportion to the difference in braking forces, and consequently short, is selected, particularly in the case of roadways with a uniformly low coefficient of friction that requires switching to select-low mode. Switching to select-low mode can also take place, in particular, after curved travel of the motor vehicle has been detected. Likewise, switching back to the first (SH) mode take place after a comparatively short second waiting time that takes the road conditions into account.